That is part of the problem -- it is a paradox. Whilst it's known in the true crime community and has even featured on some list-based websites and YouTube channels otherwise unrelated to true crime, it is still possible someone hasn't seen the photo.
It's possible that they don't remember taking the photo at all. It's even possible that both the people in the photo are dead of things totally unrelated to the photo itself. And, of course, it's totally possible that the people involved have seen it, but don't know how to come out with it after so long, or maybe just don't want to and just want to go on with their lives instead of inviting police and, as part of that, the media/internet folks, into their lives. It could even be the case that inviting police into their current lives would ruin their current lives. It's impossible to know.
The internet goes crazy trying to identify people and it's awesome when they approach it the right way and everything goes right and there's some positive outcome from their hard work. But sometimes, it goes horribly wrong and suddenly someone who was in an innocent photo x amount of years ago, or briefly went missing x amount of years ago (Joanna Lopez is who I'm thinking of in this particular case) has strangers calling them, doxing them, inviting themselves into their current lives. Or worse -- the person is dead from something unrelated but equally tragic, and their family is harassed about some dumb photo or assumption (thinking of the accusations against people after the Boston Bombings, in this case). I can think of endless reasons a person might not want to come forward even if they know of the photo.
But you're right -- the paradox is that, in theory, someone should recognise themselves, their family, or friend in a photo and have come forward at least to the police. If I say someone should have reported siblings missing, that also in theory means someone should have reported knowing these kids at the time and their status and, in these many years, should have felt able to step forward and either debunk or reinforce the nature of the photo. Then again, maybe a person saw the photo, thought it may be x person, but wasn't sure and couldn't remember enough anyway to make any report meaningful because they knew them briefly or as a childhood friend they lost contact with.
However, I feel it's also totally possible this photo has gone over the heads of a lot of people and we feel it's more well-known than it is simply because we are plugged in to the true crime community and such. Like I mentioned, it's found itself on list websites and videos -- but people into true crime are far more likely to click something called '10 Disturbing Photos Associated with a Terrible Crime!' (for example) than someone who doesn't care for true crime.
